I resisted an art party for years because of the mess. Then I moved the whole thing outside and realised the mess was the entire point — and the hose sorts it out afterwards.
The big shared canvas was the surprise. Instead of everyone doing their own small picture, they painted one enormous thing together, and it was the calmest forty minutes of the afternoon.
We cut it into squares at the end so every child took home a piece of the same painting.
Tip
Buy washable poster paint, not acrylic — the difference on clothes and fences is enormous. Have a bucket of soapy water and a stack of old towels at the edge of the painting station, and warn parents in the invitation that children will come home colourful.